> So why couldn't the info be kept in the 'SGA' along side other dynamic V$> info?

Because there is need to wake up processes. Namely, if a process is
sleeping, he's not actively reading SGA and if it has allocated memory
that is needed, the process must shrink. The process is in the sleep state,
so it doesn't know that it needs to relinquish memory. There must be some
kind of "conductor" which will send a signal and wake it up in order
to free memory.